One of the ways to improve the absorption of cephalosporin antibiotics which are poorly absorbed through the digestive tract is to prepare and administer the corresponding ester derivatives at the 4-carboxylic acid position. The esters are then readily and completely hydrolysed in vivo by enzymes present in the body to regenerate the active cephalosporin derivative having the free carboxylic acid at the 4-position.
Among the various ester groups that can be prepared and administered only a selected few are biologically acceptable, in addition to possessing high antibacterial activity and broad antibacterial spectrum. Clinical studies on many such potential “prodrug esters” such as cefcanel daloxate (Kyoto), cefdaloxime pentexil tosilate (Hoechst Marion Roussel) and ceftrazonal bopentil (Roche), to name a few have been discontinued, while ceftizoxime alapivoxil ((Kyoto) in under Phase III clinical studies. The cephalosporin prodrug esters which have been successfully commercialised and marketed include cefcapene pivoxil (Flomox®, Shionogi), cefditoren pivoxil (Spectracef®, Meiji Seika), cefetamet pivoxil (Globocef®, Roche), cefotiam hexetil (Taketiam®, Takeda), cefpodoxime proxetil (Vantin®, Sankyo), cefteram pivoxil (Tomiron®, Toyama) and cefuroxime axetil (Ceftin® and Zinnat®, Glaxo Wellcome).
Typically, such (3,7)-substituted-3-cephem-4-carboxylic acid esters represented by formula (I A) are synthesised by reacting the corresponding (3,7)-substituted-3-cephem-4-carboxylic acid derivative of formula (III A), with the desired haloester compound of formula (IV A) in a suitable organic solvent. The synthesis is summarised in Scheme-I, wherein in compounds of formula (I A), (II A), (III A) and (IV A) the groups R1 and R2 at the 3- and 7-positions of the β-lactam ring are substituents useful in cephalosporin chemistry; R3 is the addendum which forms the ester function and X is halogen.

However, the esterification reaction which essentially involves conversion of a polar acid or salt derivative to a neutral ester product invariably produces the corresponding (3,7)-substituted-2-cephem (Δ2)-4-carboxylic acid ester derivative of formula (II A) in varying amounts, arising out of isomerisation of the double bond from the 3–4 position to the 2–3 position as well as other unidentified impurities.
It has been suggested [D. H. Bentley, et. al., Tetrahedron Lett., 1976, 41, 3739] that the isomerisation results from the ability of the 4-carboxylate anion of the starting carboxylic acid to abstract a proton from the 2-position of the 3-cephem-4-carboxylic acid ester formed, followed by reprotonation at 4-position to give the said Δ2-ester. It has also been suggested [R. B. Morin, et. al., J. Am. Chem. Soc., 1969, 91, 1401; R. B. Woodward, et. al., J. Am. Chem. Soc., 1966, 88, 852] that the equilibrium position for isomerisation is largely determined by the size of the ester addendum attached at the 4-carboxylic acid position.
The 2-cephem-4-carboxylic acid esters of formula (II A) are not only unreactive as antibacterial agents but are undesired by-products. Pharmacopoeias of many countries are very stringent about the presence of the 2-cephem analogues in the finished sample of (3,7)-substituted-3-cephem-4-carboxylic acid esters and set limits for the permissible amounts of these isomers. Due to the structural similarity of the 2-cephem and 3-cephem analogues it is very difficult to separate the two isomers by conventional methods, such as chromatography as well as by fractional crystallisation. In addition to this removal of other unidentified impurities formed in the reaction, entails utilisation of tedious purification methods, thus overall resulting in,    a) considerable loss in yield, increasing the cost of manufacture and    b) a product of quality not conforming to and not easily amenable for upgradation to pharmacopoeial standards.
Several methods are reported in the prior art for synthesis of cefuroxime axetil of formula (I) and various (3,7)-substituted-3-cephem-4-carboxylic acid esters of formula (I A), with attempts to minimise the unwanted Δ2-isomers formed in such reactions as well as conversion of the Δ2-isomer thus formed back to the desired Δ3-isomer. The prior art methods can be summarised as follows:    (i) U.S. Pat. No. 4,267,320 (Gregson et. al.) describes a method for synthesis of cefuroxime axetil comprising reaction of cefuroxime acid or its alkali metal salts or onium salts with (R,S)-1-acetoxyethyl bromide in an inert organic solvent selected from N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, acetone, acetonitrile and hexamethylphosphoric triamide at a temperature in the range of −50 to +150° C. The patent mentions that when alkali metal salts, specially potassium salt of cefuroxime acid are employed the reaction can be carried out in a nitrile solvent in the presence of a crown ether. When cefuroxime acid is employed the reaction is carried out in the presence of a weak inorganic base such as sodium carbonate or potassium carbonate, which is added prior to the addition of the haloester. The patent further mentions that the use of potassium carbonate in conjunction with the haloester, specially the bromo or iodo ester is preferred since it helps to minimise the formation of the Δ2-isomer. Ideally, substantially equivalent amounts of cefuroxime acid and the base is employed.
The U.S. Pat. No. 4,267,320 also describes methods, wherein the said esterification is carried out in the presence of an acid binding agent, which serve to bind hydrogen halide liberated in the reaction, thereby controlling the formation of the Δ2-isomer. The acid binding agents that are utilised include a tertiary amine base such as triethylamine or N,N-dimethylamine; an inorganic base such as calcium carbonate or sodium bicarbonate and an oxirane compound such as ethylene oxide or propylene oxide.
However, from the examples provided in the above patent the yield of cefuroxime axetil and other (3,7)-substituted-3-cephem-4-carboxylic acid esters obtained is found to be only of about 50%, implying formation of substantial amounts of impurities in the reaction. Indeed, when cefuroxime acid is reacted with (R,S)-1-acetoxyethyl bromide in the presence of 0.55 molar equivalents of sodium carbonate or potassium carbonate in N,N-dimethylacetamide as solvent, as per the process disclosed in this patent, it is found that substantial amounts of the Δ2-isomer in a proportion ranging from 10–22% is formed, in addition to other unknown impurities. Also, substantial amounts of the starting cefuroxime acid remains unreacted even after 5 hrs of reaction. Isolation of the product generally affords a gummy material, which resists purification even after repeated crystallisations.
Moreover, the use of the acid binding agents mentioned in the above patent, specially tertiary amines and inorganic bases lead to cleavage of the β-lactam ring and also promote the undesired Δ2-isomerisation, thereby enhancing the level of impurities formed in the reaction.    (ii) (GB Patent No. 2 218 094 describes a method by which the Δ2-isomers formed during esterification can be converted back to the desired Δ3-isomers. The method comprises of oxidation of the dihydrothiazine ring in the mixture of Δ2- and Δ3-cephalosporin acid esters to the corresponding sulfoxide derivatives with suitable oxidising agents, whereby the Δ2-isomer gets isomerised to the corresponding Δ3-isomer during oxidation and the Δ3-cephalosporin acid ester sulfoxide is isolated. The sulfide group is regenerated back by reduction of the sulfoxide function with suitable reducing agents.
Typically, the oxidation is carried out using m-chloroperbenzoic acid and the reduction achieved by use of an alkali metal halide in presence of acetyl chloride in presence of an inert organic solvent or by use of a phosphorous trihalide.
Although, this method provides the desired Δ3-isomers in good purity, it cannot be considered as an industrially feasible method since it involves a two step process of oxidation and reduction, isolation of the intermediate products at each stage and necessary purifications, all resulting in considerable loss of the desired product and increase in the cost of manufacture. Moreover, the use of acetyl halide and phosphorous trihalide in the reduction step cannot be applied to cephalosporin derivatives that are sensitive to these reagents.
A similar method has been reported by Kaiser et. al. in J. Org. Chem., 1970, 35, 2430.    (iii) Mobasherry et. al. in J. Org. Chem., 1986, 51, 4723 describe preparation of certain Δ3-cephalosporin-4-carboxylic acid esters by reaction of the corresponding 3-cephem-4-carboxylic acids (in turn prepared form the corresponding carboxylic acid alkali metal salts) with an haloester in presence of 1.1 eq of sodium carbonate in the presence 1.2–1.5 eq of an alkyl halide and in presence of a solvent comprising of a mixture of N,N-dimethylformamide and dioxane. The authors claim that the method provides of Δ3-cephalosporin-4-carboxylic acid esters unaccompanied by the corresponding Δ2-isomer.
However, the method involves an additional step in that the starting 3-cephem-4-carboxylic acid ester derivatives are obtained from the corresponding alkali metal salts prior to reaction. In addition, longer reaction times of about 24 hrs coupled with the fact that it utilises dioxane, a potent carcinogen, not recommended by International Conference on Harmonisation (ICH) on industrial scale renders the method unattractive commercially.
Moreover, on duplication of the method exactly as described in the article it is found that about 3–4% of the corresponding Δ2-isomer is indeed formed in the reaction in addition to other unidentified impurities. Also, substantial amounts of the starting cephalosporin carboxylic acid is recovered unreacted.    (iv) Shigeto et. al. in Chem. Pharm. Bull., 1995, 43(11), 1998 have carried out the esterification of certain 7-substituted-3-cephem-4-carboxylic acid derivatives with 1-iodoethyl isopropyl carbonate in a solvent system containing a mixture of N,N-dimethylformamide and dioxane in a 3:5 ratio. A conversion to the corresponding 3-cephem-4-carboxylate ester was achieved in only 34%, out of which the Δ2-isomer amounted to about 8%.
Esterification of 7-formamido-3-(N,N-dimethylcarbamoyloxy)methyl-3-cephem-4-carboxylic acid sodium salt with a suitable haloester in presence of solvents such as N,N-dimethylacetamide and N,N-dimethylformamide, with formation of about 0.8 to 3.0% of the Δ2-isomer is also reported in the above article by Shigeto et. al. The 7-formamido group was cleaved under acidic conditions to give the corresponding 7-amino derivative contaminated with only about 0.4% of the corresponding Δ2-isomer. The minimisation of the percentage of Δ2-isomer is attributed to the relative unstability of 7-amino-2-cephem-4-carboxylic acid esters in acidic conditions, facilitating isomerisation of the 2-cephem intermediate to the 3-cephem derivative.
However, the method does not have a general application, especially for synthesis of commercially valuable cephalosporin derivatives containing hydroxyimino or alkoxyimino substituents in the 7-amino side chain addendum, since these oxyimino functions exhibit a tendency to isomerise from the stable (Z)-configuration to the relatively undesirable (E)-configuration under acidic conditions. This would render separation of the two isomers cumbersome. Moreover, longer reaction times of about 18–20 hrs to effect the isomerisation of the double bond from the 2-position to the 3-position and use of toxic dioxane as solvent impose further limitations on the method.    (v) Demuth et. al. in J. Antibiotics, 1991, 44, 200 have utilised the N,N-dimethylformamide-dioxane system in the coupling of 1-iodocephem-4-nitrobenzyl ester with naldixic acid sodium salt and recommend use of dioxane since it reduces the basicity of the quinolone carboxylate and lowers the polarity of the reaction medium.
However, low yields of about 35% and use of toxic dioxane makes the method of little industrial application.    (vi) Wang et. al. in U.S. Pat. No. 5,498,787 claim a method for preparation of certain (3,7)-substituted-3-cephem-4-carboxylic acid prodrug esters, unaccompanied by the analogous 2-cephem esters comprising reaction of the corresponding (3,7)-substituted-3-cephem-4-carboxylic acid alkali metal salts with suitable haloesters in the presence of catalytic amounts of a quarternary ammonium or quarternary phosphonium salt. Among the prodrug esters covered in this patent is cefuroxime axetil.
U.S. Pat. No. 5,498,787 claims that among the quarternary ammonium salts, such salts with acid counter ion, specially tetrabutyl ammonium sulfate (TBA+HSO4−) is the most preferred. When the molar ratio of TBA+HSO4−/cefuroxime sodium was above 0.40 no Δ2-isomer was detected, when the said molar ratio was below 0.40 and near about 0.20 the molar ratio of Δ2/Δ3 isomers formed was about 2.0%. When no TBA+HSO4− was added the molar ratio of Δ2/Δ3 isomers formed was about 10.0%. Examples 1 and 2 of this patent illustrate the esterification of cefuroxime sodium in presence of TBA+HSO4− and indicate that the Δ2-isomer was not detected after 3–12 hours of reaction. The same patent also establishes the superiority of TBA+HSO4− over other salts, specially tetrabutyl ammonium iodide (TBA+I−) since use of the latter salt resulted in considerable isomerisation of the double bond giving the undesired Δ2-isomer in predominant amounts.
The present inventors have, however, found that when cefuroxime sodium is reacted with (R,S)-1-acetoxyethyl bromide in the presence of tetrabutylammonium sulfate (TBA+HSO4−) as per the method covered in U.S. Pat. No. 5,498,787 the same did not necessarily result in the production of the desired Δ3 isomer free of the undesired Δ2 isomer and other impurities. Also, such process had limitations in that the reaction could not be completed at times even at the end of 5.0 hrs. Moreover, the separation of the impurities, from the product proved cumbersome and could not be removed from the product even after successive crystallisations.    (vii) H. W. Lee et. al., Syntheic Communications, 1998, 28(23), 4345–4354 have demonstrated a method essentially similar to that claimed in U.S. Pat. No. 5,498,787. The method of preparation of various esters of cefotaxime consists of reacting cefotaxime sodium with the requisite haloester compound in a suitable solvent and in presence of quarternary ammonium salts as phase transfer catalysts. It is claimed that when no quarternary ammonium salts are added the molar ratio (%) of Δ2/Δ3 isomers formed is about 10%. The formation of Δ2-isomer is minimised when quarternary ammonium salts are added and particularly when the molar ratio of TBA+HSO4−/cefotaxime sodium employed is 0.80 the formation of the Δ2-isomer is completely inhibited.
However, this method requires long hours (˜18–24 hrs) and is carried out at higher temperatures (40–45° C.) and as such may not be suitable for cephalosporin derivatives that are sensitive to heat.    (viii) H. W. Lee et. al. in Synthetic Communications, 1999, 29(11), 1873–1887 demonstrate a method for preparation of number of (3,7)-substituted-3-cephem-4-carboxylic acid esters comprising reacting the corresponding (3,7)-substituted-3-cephem-4-carboxylic acid derivatives with a base selected form cesium carbonate or cesium bicarbonate either used alone or in combination with potassium carbonate, sodium carbonate, potassium bicarbonate and sodium bicarbonate. The authors established that the formation of Δ2-isomers could be minimised by utilisation of a solvent combination of N,N-dimethyl formamide and dioxane. The use of the latter mentioned solvent i. e. dioxane was expected to lower polarity of the reaction medium and thereby reduce the basicity of the transient 3-cephem-4-carboxylate anion formed in the reaction and thus preventing the isomerisation of the double bond from the 3–4 position to the 2–3 position.
The formation of the Δ2-isomer was found to be dependent on the amount of dioxane in the solvent mixture, the more the proportion of dioxane lesser the degree of isomerisation.
However, yields of representative esters obtained by the method are in the range of 45–85%, implying that the reaction is accompanied by formation of substantial amounts of impurities and that the isomerisation is dependent on the nature of the substituent at 3α-position of the cephalosporin nucleus as well as on the nature of the haloester employed. Moreover, the method utilises dioxane, not desirable for reasons mentioned herein earlier and expensive cesium salts. This method, therefore, also has limited application.    (ix) Y. S. Cho et. al., in Korean J. Med. Chem., 1995, 5(1), 60–63 describe synthesis of several cephalosporin prodrug esters and their efficacy on oral administration. The esters were synthesised by reacting the corresponding cephalosporin-4-carboxylic acid derivative with the respective haloester derivative in presence of cesium carbonate and N,N-dimethylacetamide. The yields of the ester derivatives obtained are in the range of only 25–56%, indicating formation of substantial amounts of impurities in the reaction.
Thus, in summary the prior art methods are associated with one or more of the following shortcomings, which limit their application as an industrially acceptable method for synthesis of various (3,7)-substituted-3-cephem-4-carboxylic acid esters, specially cefuroxime axetil. These are, viz.    a) formation of varying amounts of the undesired 2-(Δ2)-cephem-4-carboxylic acid esters, and other unidentified impurities, specially impurities X1 and X2 mentioned earlier,    b) incompleteness of the reaction, resulting in substantial amounts of starting material remaining unreacted,    c) employment of tedious/costly techniques for separation of the unwanted Δ2-isomer, other unidentified impurities and unreacted starting material,    d) shortcomings a), b) and c) giving the final product in low yields and of inferior quality, thereby making the methods commercially unviable,    e) use of carcinogenic and toxic solvents not acceptable industrially,    f) use of additives/catalysts as acid binding agents not efficient enough to prevent the Δ3- to Δ2-isomerisation and    g) lack of general applicability for synthesis of a variety of (3,7)-substituted-3-cephem-4-carboxylic acid esters.
It is thus the basic object of the present invention to provide for an improved process for manufacture of (3,7)-substituted-3-cephem-4-carboxylic acid esters particularly cefuroxime axetil of formula (I) which would be substantially free of undesired 2-(Δ2)-cephem-4-carboxylic acid esters and any associated impurities.
It is another object of the present invention to provide for an improved synthesis of (3,7)-substituted-3-cephem-4-carboxylic acid esters, particularly cefuroxime axetil of formula (I), which eliminates/minimises the aforesaid shortcomings associated with the prior art methods and provides the object compound(s) in highly pure form, suitable for use in pharmaceuticals.
Another object of the present invention is to provide compounds of formula (I) in high purity i.e. of quality conforming to pharmacopoeial standards.
Yet further object of the present invention is to provide a cost-effective and environmentally benign method for preparation of cefuroxime axetil of formula (I) in high purity utilising cost effective and readily available raw materials and industrially acceptable solvents.